Optical particle counters (OPCs) are used to detect and measure the size of individual particles suspended in a fluid. Generally, particle counters are designed to detect particles of less than one micron in size. Each particle that is detected is counted, and an indication of the number of particle counts within a channel, with each channel corresponding to a particular size range, is provided. For particle counters to operate effectively, the density of particles in the fluid must be small enough that the particles are considered to be contaminants in the fluid. That is, it is important to distinguish the science of particle counting from other scientific fields, such as photometry and cytometry, which also utilize scattered light, but in which the density of the particles in the fluid is so large that they are essentially the fluid itself. These latter systems rely on collecting scattered light from millions, billions, and more particles; therefore, their principles of operation are very different from the principles used in particle counters.
Particle counters are commonly used to detect contaminant particles in the fluids used in manufacturing clean rooms in the high-tech electronics and the pharmaceutical industries. Generally, it is not possible to detect all the particles in the fluid of interest, since the amount of fluid is quite large. Thus, small samples of the fluids used in the manufacturing processes are diverted to the particle counters, which sound an alarm if the number and/or size of the particles detected is above a predetermined threshold. Since a small sample of the manufacturing fluid is generally not completely representative of the entire volume of the manufacturing fluid, statistics are used to extrapolate the state of the manufacturing fluid from the sample. The larger the sample, the more representative it is, and the more quickly an accurate determination of the number and size of particles in the manufacturing fluid can be made. It is desirable for a particle counter to detect particles as small as possible, as fast as possible, in as large a sample as possible.
FIG. 1 shows a schematic depiction of the optical design of a conventional liquid OPC 100. The laser beam 110 passes through windows 111 and 113 of a sample cell 116 and is focused to a narrow waist 112 in the liquid flow 114 of the sample cell 116. In this cell, the liquid flows in a direction out of the paper. The very high laser irradiance (power per area) at the laser beam waist 112 is a key element in being able to detect particles less than 100 nm in diameter. The laser radiation scattered by each particle is focused onto a detector 140 through a large numerical aperture, complex imaging system 150. The use of high power illumination generally enhances particle detection. Specifically, higher power levels generally enable the detection of smaller particles than lower power systems. Higher power levels also generally permit particles of a given size to be detected more quickly. Thus, high power lasers are generally used as the light source in particle counters.
Another constraint that must be taken into account in particle counting is the background noise due to scattering of light by the molecules of the fluid in which the contaminant particles are contained. While the amount of light scattered by each molecule of fluid is small, the number of fluid particles is extremely large. For example, there are approximately 1016 air molecules per cubic mm at atmospheric pressure. The light scattered by the fluid molecules cannot be eliminated. This light creates a background of molecular scattering noise that masks the signal from the contaminant particles.
In U.S. Pat. No. 4,798,465 issued Jan. 17, 1989 to Robert G. Knollenberg, which patent is hereby incorporated by reference, it was demonstrated that the background molecular scattering noise can be reduced by use of a one-dimensional detector array. When the sample volume is imaged onto such a detector array, the molecular scatter noise is divided by the number of elements. However, if the detector optics are correctly designed, the scattered light signal from the particle is imaged onto one detector element. Since this invention, the state-of-the-art OPC typically has employed a twenty-element array. The net result is that the use of a twenty-element detector array, in place of a single-element detector, yields the same signal-to-noise benefit as increasing the laser irradiance by twenty-fold.
U.S. Pat. No. 5,282,151 issued Jan. 25, 1994 to Dr. Robert Knollenberg, which patent is hereby incorporated by reference, proposed the use of a high-density two-dimensional CCD (Charge Coupled Device) detector. Knollenberg proposed that in this design, like the one-dimensional array design, the signal-to-noise advantage theoretically should be proportional to the number of detector elements. Since CCD arrays typically contain greater than 1,000,000 elements, CCD arrays would appear to offer a promising design option for boosting OPC performance. However, the design was not successful and never became a commercial product. Fundamentally, the geometry of the conventional optical particle counter as shown in FIG. 1 did not allow for the effective use of two-dimensional detector arrays. Thus, for the last decade and a half, improvements in liquid OPC performance, at particle sizes less than 100 nm, have been achieved through the use of higher powered, more expensive lasers.
The users of particle counters, such as high-tech electronics manufacturers, are continually making advances that permit smaller and smaller device parameters. Modern semiconductor chips are complex three-dimensional structures of transistors and other electrical components. Particles in de-ionized water (DIW) and other process fluids can create defects by clinging to wafers, thus interfering with photolithography, as well as physical and chemical vapor deposition processes. The purity of DIW is particularly important, because DIW is used as the final rinse in most fabrication processes before proceeding to the next process. The prevailing opinion in the semiconductor industry is that the maximum allowable diameter of particulate contaminants in DIW equals one-half the semiconductor line width. The International Technology Roadmap for Semiconductors (ITRS) specifications, consistent with this guideline, for allowable particulate contamination in DIW are given in Table A, where hp90 and hp65 refer to 90 nm and 65 nm linewidths, respectively. Over the last two years, conventional optical particle counter (OPC) designs have been hard pressed to keep pace with the ITRS DIW sensitivity specifications, and conventional designs have no hope of achieving future targets.
TABLE 1ITRS Particle Contamination Specifications for DI WaterProduction Year2003200420052006200720082009Technology Nodehp90hp65Critical Particle Size50 nm45 nm40 nm35 nm33 nm29 nm25 nmParticle Concentration<0.2/ml<0.2/ml<0.2/ml<0.2/ml<0.2/ml<0.2/ml<0.2/ml
Therefore, there exists no known, practical, liquid OPC design which can provide a significant increase in sensitivity over the conventional design depicted in FIG. 1 using the array described by Knollenberg in U.S. Pat. No. 4,798,465. Furthermore, this conventional liquid OPC design is unable to keep pace with the International Technology Roadmap for Semiconductor given in Table 1.
Accordingly, there is a need in the art for a particle counter system and method that can more effectively process out molecular background noise and permit smaller contaminant particles to be detected. In particular, there is a need in the art for a particle counter system that is able to effectively utilize a sensing array larger than twenty array elements.